the alkaline glycine process is a relatively new method for extracting metals from both natural ores and industrial waste streams. it is used both in conjunction with depleted cyanide and on its own.
the selectivity of alkaline glycine extraction when processing a mixed input stream is based primarily on temperature. the solution of glycine at or above a pH of 9 with the addition of 1% H2O2 is consistent. the solution will put copper, iron, and nickel into solution at room temperature, and when heated to 60C will also complex and dissolve silver and gold.
selective precipitation of the recovered metals is done by various methods, and is still a subject of experimentation for my personal processes, as the industrial scale typically uses very specific organic solvents to extract the copper from solution in a biphasic mix which is then separated, leaving a solution of primarily precious (PM) metal glycine complexes. once this is achieved, the PMs are then precipitated out of solution either via cementation on zinc (or possibly other reactive metals like aluminum), or on activated carbon (i have seen figures in the realm of 3g/L of solution).
the method for copper precipitation that i am hoping to incorporate into my process is that of using sodium hydrosulfide (NaSH), which would precipitate insoluble copper sulfide, or possibly using a copper electrowinning cell. Either of these methods should ideally preserve the glycine ligands, allowing the solution to be reused for more extraction of mixed metal input material.
below are some (admittedly poor) photos of the process of using an alkaline glycine solution to dissolve the copper and nickel layers of hard-gold plated fingers from the card edge connectors on various PCBs.
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